The microbial chemistry of bacteriophage lambda DNA replication in E. coli bacteria has been extensively studied and constitutes one of three or four highly developed model systems for DNA replication. Previous research by the principle investigator has demonstrated the existence of a rolling circle mechanism which has two unusual features -an apparent arrest in further replication which may be due to the establishment of lysogeny and a crosslink which connects l and r strands and which results in spontaneously renaturable DNA. The cross-link may arise at the replication fork. Experiments are proposed to map the site of the crosslink which may occur at the origin (terminus) of DNA replication, to locate the crosslink physically on the rolling circle DNA structure, and to determine the relationship, if any, between the crosslink, the arrest of further DNA replication involving parental label, and the establishment of lysogeny.